1. Field of the Invention
The invention provides adhesives for the releasable attachment of substrates, in particular glass-like plastic substrates, to thermoplastic or duroplastic plastic substrates, and the use of same for the releasable attachment of light covers to lamp housings of automobile lights or automobile headlights, as well as automobile lights or automobile headlights produced using said adhesives.
2. Description of the Related Art
The technology relates to a joint between first and second assembly parts having adjoining side walls, which may be tightly joined together by means of a sealing bed that may be provided with a removable adhesive sealant.
The technology further relates to a method for producing a joint between first and second assembly parts, whereby a removable adhesive sealant in a liquid state is applied to a sealing bed area of the first assembly part, and both of the assembly parts are joined together.
In particular for automobile headlights, joints are known where a first assembly part, such as a headlight housing, has a U-shaped sealing bed on a first side wall into which a second assembly part, such as a closure disk or cover disk, having a second side wall is insertable, such that both of the assembly parts may be joined with a seal.
A disadvantage with the known joints is that, for use of such U-shaped sections, the adhesive sealant cannot be easily withdrawn upon removal, since undercuts and corners present may pose the danger of tearing-off. In order to remove the adhesive sealant, both assembly parts must be disassembled beforehand, and the adhesive sealant must be removed with great effort.
As a result of the increasing interest in the recycling the components of used products, there is a need for attachment techniques that allow easy assembly and disassembly.
A known joint seal is described in DT 1 804 736 where the side wall of a first assembly part has a U-shaped sealing bed, into which the side wall of a second assembly part is inserted. After joining together of same, an adhesive sealant is introduced into the sealing bed via lateral openings. The disadvantage here is that the adhesive sealant is very difficult to remove without leaving residue.
Known hot-melt adhesives can be produced in cartridges or barrel-like cylinders, or as slab-stock blocks (separated from each other by adhesive-coated paper), for example. When cartridges are used, frequently the entire quantity of adhesive is heated and processed using a hand applicator. For larger barrel-like cylinders, a heated stamp in a stationary arrangement, with removal and delivery lines, is placed on the hot-melt adhesive surface, and removed only from the surface area. In this manner, only a portion of the hot-melt adhesive is heated. Since the hot-melt adhesive is force-fed, there are no strenuous requirements on the flow characteristics this type of application technique.
An associated disadvantage is insufficient repeat releasability and residual adhesion of the hot-melt adhesive to the substrates after they are separated.
In the current art, products from Beiersdorf AG, Hamburg, under the description “powerstrips,” are known that represent releasable but pressure-sensitive adhesive attachments in the sense of the present invention.
The pressure-sensitive adhesive strip can be removed by pulling on a handle in the direction of the adhesive plane. The substrates and adhesive strip are recovered and the materials separated (DE 43 39 604 A and DE 33 31 016 A).
U.S. Pat. No. 4,024,312 also describes the principle based on the adhesive strip. This patent is follows others, having the common feature that an adhesive strip comprising a pressure-sensitive adhesive (PSA), applied at ambient temperature, is exclusively used, which is preferably provided with a strongly stretchable, tear-resistant reinforcement backing. This backing is necessary, as described in the adhesive strip description, to prevent tearing off of the adhesive film before final removal from the adhesive joint. Since flat shapes are hereby attached to one another, tearing off of the adhesive strip is very likely, resulting in inconvenience to the user, as is described in DE 43 39 504 A.
U.S. Pat. No. 4,009,793 describes a releasable attachment between a lid and a storage container by using a heat-activated shrinking adhesive strip with simple overlapping on the ends. The adhesive strip is wrapped around the area to be sealed, and the shrinking process is thermally induced. The adhesive strip is provided with a reinforcement backing to assure repeat releasability. A particular disadvantage of this method is that it is impossible to tightly bond different objects to one another if the surface contour changes from a convex to a concave curved shape. A further disadvantage lies in the fact that a customary adhesive strip must be removed with application of great force to effect an integral separation of the individual components. Accordingly, the last item was not achieved by this patent.
Adhesives allowing releasable attachments are also described in U.S. Pat. No. 4,305,996. A polyol is required for polyurethane-based materials, which permits the desired low viscosities when contact adhesives are formulated with said polyol. The PSA described in said patent is designed to be removed repeatedly at a peel-off angle of 180°. The production of an adhesive strip thus necessarily conforms to this requirement. A PSA is prepared as an adhesive strip before the actual joining process, and is attached to the substrates by application of pressure on the adhesive strip.
Another technique to seal joined housing parts is described in U.S. Pat. No. 4,775,076. This method is suitable only with the limitation that the housing parts to be joined must have cylindrical shape in the joint area. At ambient temperature, flexible sealant is cut to length and inserted into the cylindrical joint area of the first substrate, the second substrate with its cylindrical surface is placed against said first substrate. By external action on the joint area, the sealing material is brought to the melting point by increasing the temperature and applying pressure to assure bonding or adhesion of the substrates to or onto one another. This operation does allow release, but not an integral separation.
Further proposals for releasable attachments of a plurality of substrates, in the form of adhesive strips, among others, are described in U.S. Pat. No. 5,897,949 as a PSA adhesive strip with foamed reinforcement backings; in U.S. Pat. No. 5,827,591 as a PSA-laminated paper; in U.S. Pat. No. 5,672,402 as a PSA adhesive strip having a non-elastic reinforcement backing in the embodiment; and in U.S. Pat. No. 5,470,622 with an artificially aged adhesive having a reinforcement backing that is shrunk using temperature.
Other examples of releasable attachments of a plurality of substrates to one another have come from the field of lithographic printing, whereby the layer creating the imprint is designed to be removable from the calender roll, which is arranged as a cylindrical core, to enable print copy to be exchanged for economy of material and time. In U.S. Pat. No. 5,870,955 it is proposed, after completion of printing of the desired number of print copies, that the actual print layer from the foundation, that is, the core and adhesive, be peeled off and discarded. In order to release the print layer from the foundation, it is proposed that heating the entire structure allows elimination of the internal bond and separation of the attachment. Care must be taken that the acrylate adhesive itself is tearable, and that adhesion to the core is achieved. The adhesive is applied to said roll as a film.
U.S. Pat. No. 4,461,663 describes a corresponding method for a releasable press-on layer of a roller core. Releasablity is effected by heating the entire structure, and the upper press-on layer is separated from the foundation by peeling off. This adhesive is applied as a hot melt. In this type of separation procedure, the hot melt becomes soft and may lose its cohesiveness.
In the past, it has been proposed to use gels—made, for example, from silicones used in potting applications in the electronics field—for releasable sealants for headlight systems in the automotive field. However, four main disadvantages have been identified. These systems are applied in the liquid state into a standard sealing bed, which heretofore has been customarily U-shaped for headlight systems, and must be cured for some time either before or after joining, and additional attaching elements are necessary. Since silicone-based materials do not have good adhesion to polypropylene without pretreatment, a further disadvantage results from this special application. The attachment is not secure against penetration by a concentrated water jet, such as from a high-pressure cleaner. Quantities of a mobile silicone softening oil are present, as is also described in U.S. Pat. No. 5,886,111. This latter disadvantage must be eliminated if materially corresponding systems of other designs are used.
This type of sealant design enables the joint parts to be released from one another by pulling the substrates apart.
Also known in the current art is a headlight system from Ichikoh, Japan, wherein a gel-like material that easily adheres to the surface is used. This system has a very low modulus in addition to high reversion capability, and is applied in a heretofore “classical” U-shaped sealing bed of an automobile headlight. After a hardening process, the base of the closure disk is pressed into the material, and is held in this position by means of a number of circumferentially arranged fastening elements. This material allows the sealing disk to be manually removed, although with application of great force, from the attachment after removal of the safety elements. To this end, both substrates themselves must be pulled. A further disadvantage of this system lies in the fact that a narrowly concentrated water jet from a high-pressure cleaner, shooting through the attachment between sealant and substrate, cannot be prevented when this area is affected. In the construction design of said system, a rubber sealant must be applied in the liquid state and held under constant pressure to assure a positive supported sealing function from the slight adhesion.
In order to achieve the object of a releasable attachment of a sealing disk to a headlight housing, U.S. Pat. No. 5,560,706 describes a non-adhesive sealant that is foamed into the U-shaped adhesion bed. The seal tightness is assured according to the invention by sufficiently high pressure forces, which are achieved by means of mechanical clamping systems. To this end, the foamed material must be cured before joining, thus requiring considerable time and apparatus in the form of a sufficiently long curing process. At an ambient temperature of approximately 100° C., said foamed sealant experiences stress relaxation and the headlight becomes loosened, thus representing a further disadvantage. After the article thus produced is used, the headlight can be opened after being removed from the clamps.
As a result, there is a great need for an adhesive substance that can be used on variously shaped substrates in a commercial production process, and that permits a residue-free, simple release of the attached components from one another after use. As an example of this ever-increasing need, automobiles in the future will have to be capable of virtually complete disassembly and material recycling, although the hitherto increasing prevalence of this adhesion method has been classified as non-releasable. The various releasable attachment techniques cited above all have significant disadvantages. Releasable adhesive strips are unsuitable in commercial production for surfaces with complex shapes, with the typically occurring gap tolerances and high handling expense. Meltable hot-melt adhesive substances, which permit softening and subsequent cohesive failure of the adhesive, are likewise unsatisfactory due to the heating step and residual adherence of the adhesive. In particular, for large-surface adhesion or sealing with typically small surface loads, directly applicable adhesives or adhesive sealants are very advantageous, especially for residue-free removal.
Such applications could include adhesive sealants for inner paneling, splash guard flaps, and bumpers for automobiles, as well as frame and window sealants in the construction and automotive fields, vibration- and sound-damping silencers, and other similar commercial applications.
In general, all lighting devices for automobiles, produced using hot-melt adhesive sealants, are classified as releasable. To this end, the entire structure must be heated, and the substrates are separated by pulling away from one another. A residue of adhesive sealant thus typically remains on the substrates.